In the course of assessing and producing hydrocarbon bearing formation and reservoirs, it is important to acquire knowledge of formation and formation fluid properties which influence the productivity and yield from the drilled formation. Typically, such knowledge is acquired by “logging” operations which involve the measurement of a formation parameter or formation fluid parameter as function of location within the wellbore. Formation logging has evolved to include many different types of measurements, including those based on acoustic, electro-magnetic or resistivity, and nuclear interactions, such as nuclear magnetic resonance (NMR) or neutron capture.
NMR measurements are commonly used in the wellbore to probe the NMR decay behavior of the stationary fluid in the reservoir rock. During these measurements, magnetic fields are established in the formation using suitably arranged magnets. The magnetic fields induce nuclear magnetization, which is flipped or otherwise manipulated with on-resonance radio frequency (RF) pulses. NMR echoes are observed, and their dependence on pulse parameters and time is used to extract information about the formation and the fluids in it.
In particular, NMR has been used in the oilfield industry to obtain information and parameters representative of bound fluids, free fluids, permeability, oil viscosity, gas-to-oil ratio, oil saturation and water saturations. These parameters can be derived from measurements of spin-spin relaxation time, often referred to as T2, spin-lattice relaxation time (T1), and self-diffusion coefficient (D) of the molecules containing hydrogen contained in formation fluids.
On the other hand, fluids are routinely sampled in the well bore with the help of formation testers or formation fluid sampling devices, such as Schlumberger's MDT, a modular dynamic fluid testing tool. Such a tool may include at least one fluid sample bottle, a pump to extract the fluid from the formation or inject fluid into the formation, and a contact pad with a conduit to engage the wall of the borehole.
With the pumping, a flow in the formation is induced by extracting fluid from the formation through the conduit. The fluid flowing through the tool is analyzed in situ using electrical, optical or NMR based methods. Typically, when the fluid is assumed to be ‘pure’ reservoir fluid, i.e., when having acceptable levels of mud or other contaminants, a sample of the fluid is placed into the sample bottle for later analysis at a surface laboratory. The module is then moved to the next region of interest or station.
Fluid flow into the borehole is also routinely produced using dual packer arrangements which isolate sections of the borehole during fluid and pressure testing. By reversing the flow direction, dual packer arrangements offer the possibility of conducting fracturing operations which are designed to fracture the formation around the isolated section of the borehole.
It is further well established to mount logging tools on either dedicated conveyance means such as wireline cables or coiled tubing (CT) or, alternatively, on a drill string which carries a drill bit at its lower end. The latter case is known in the industry as measurement-while-drilling (MWD) or logging-while-drilling (LWD). In MWD and LWD operations, the parameter of interest is measured by instruments typically mounted close behind the bit or the bottom-hole assembly (BHA).
Applications and measurements designed to exploit the flow generated by tools such as the above formation testing tools in combination with NMR type measurements are described in a number of documents. One example is U.S. Pat. No. 7,180,288. Other NMR-based methods for monitoring flow and formation parameters can be found in U.S. Pat. Nos. 6,642,715 and 6,856,132. A tool which combines a fluid injection/withdrawal tool with a resistivity imaging tool is described in U.S. Pat. No. 5,335,542. Borehole tools and methods for measuring permeabilities using sequential injection of water and oil is described in U.S. Pat. Nos. 5,269,180 and 7,221,158. Also, in U.S. Pat. No. 5,497,321 details a method to compute fractional flow curves using resistivity measurements at multiple radial depths of investigation.
In a paper prepared for presentation at the SPWLA 1st Annual Middle East Regional Symposium, Apr. 15-19, 2007, authors Cassou, Poirier-Coutansais, and Ramamoorthy demonstrate that the combination of advanced-NMR fluid typing techniques with a dual-packer fluid pumping module can greatly improve the estimation of the saturation parameter in carbonate rocks. The ability to perform 3D-NMR stations immediately before and after pump-outs yields both the water and oil saturations (Sw,Sxo) independently of lithology, resistivity, and salinity, in a complex carbonate environment.